For each scientist you need to know Approximate date of discovery Discovery Experiment/Theory Atomic Structure from their

perspective

FOCUS

What do you know about Chemistry?

ATOMIC THEORY

Democritus – 440 BCE

Greek philosopher Credited with the discovery of the atom Believed that if you continue to cut

matter that eventually you would end up with a particle that cannot be cut

Most people did not believe this theory

Democritus

Atom came from the word atomos which means “not able to be divided”

Thought atoms were all made of a single material formed into different sizes and shapes

Atom was a small, hard, particle

John Dalton - 1803

Created original Atomic Theory All substances are made of atoms Atoms are small particles that cannot be

created, divided, or destroyed Atoms of the same element are exactly

alike and atoms of different elements are different

Atoms join together with other atoms to make new substances

John Dalton

No new atom picture Was the first to organize elements

according to mass

William Crookes - 1870

Developed the Crookes Tube, which helped him test Cathode Rays Showed that rays travel in straight lines Thought the rays were made up of ordinary

particles Doesn’t add to atomic theory, so is he

important???

JJ Thomson - 1897

Continued working with Cathode Ray Tubes (developed by Crookes) A glass tube with metal plates on each end

(cathode and anode) that is hooked up to a power source

Electric beam goes through tube When plates were not charged, the beam

went straight When plates were charged, the beam bent

toward the positively charged plate What does that tell you about the beam?

JJ Thomson

Since the beam bent towards the positively charged plate, Thomson discovered that there must be a negative charge in the atoms (neg. and pos. charges attract)

Credited with the discovery of ELECTRONS!

JJ Thomson

Plum Pudding Model

Changed model of the atom from a solid sphere to a model that had electrons mixed throughout the entire atom, like plums in a pudding

Electrons are able to come out of the atom leaving behind a positively charged area

Ernest Rutherford - 1909

Gold Foil Experiment Beam of small, positively charged particles

aimed at a thin sheet of gold foil with a coating on it that would allow him to see where the particles hit

Expected particles to pass through the foil in a straight line, and most of them did

Some particles were deflected or bounced straight back

Ernest Rutherford

Why did most of the particles go straight through the foil? Atoms are mostly empty space, so the positive

particles would simply go through that empty space

Why did some of the particles bounce back? Rutherford discovered that there must be a

dense positive charge in the middle of an atom (nucleus) and the positive charges were occasionally hitting that small, dense area

Credited with the discovery of the NUCLEUS!!

Rutherford’s Model

Center of the atom is small, dense, positive area called the nucleus with electrons surrounding in

Niels Bohr - 1913

Studied the way that atoms react to light Proposed that electrons move around

the nucleus in certain paths or energy levels

Niels Bohr

The energy levels are like a ladder

There are no paths between the energy levels, but atoms can jump from one level to another, just like you can step from one rung of the ladder to the next

Atoms cannot be in between levels, just like you cannot stand in between rungs of the ladder

Modern Atomic Theory

Electrons do not move in definite paths The path of an electron cannot be

predicted There are regions inside of the atom

where electrons are likely to be found called electron clouds

To Recap…

http://www.youtube.com/watch?v=07yDiELe83Y